This invention relates to an optical unit, a photoelectric switch, and a fiber-type photoelectric switch each comprising a plurality of optical elements, and a color discrimination sensor for detecting color of an object.
Hitherto, an optical unit such as a photoelectric switch has been used for emitting light to an object and receiving the light reflected on the object or the light penetrating the object, thereby detecting information on the object. With the photoelectric switch, light is emitted to a transport passage of an object and the presence or absence, the shape, the dimensions, the color, etc., of the object can be detected on the basis of the light reception quantity of the reflected light or penetrating light.
FIG. 11 is a sectional view to show an example of a photoelectric switch in a related art. In the figure, a light emission element 31 and a light reception element 32 are placed in a housing 35. Light emitted from the light emission element 31 is transmitted through a projection lens 33 to a detection position. Light from the detection position is gathered through a light reception lens 34 at the light reception element 32. If an object 37 exists at the detection position, light reflected from the object 37 is received on the light reception element 32 through the light reception lens 34. Therefore, whether or not the object 37 exists can be determined on the basis of the light reception quantity level of the light reception element 32.
A color discrimination sensor, which is also a kind of photoelectric switch, comprises a light transmission section containing a light source consisting of three light emission elements for respectively generating lights having wavelength bands corresponding to Red, Green and Blue and a projection lens for transmitting light emitted from each light emission element to an object and a light reception section having a detection light reception element for receiving light reflected from the object.
Light emitted from each of the three light emission elements is transmitted through the projection lens to the object in order and reflected light is received on the detection light reception element, then the color of the object can be determined on the basis of the light reception quantity level of each color band, for example, from comparison with reference color.
FIG. 12 is a sectional view of the main part of an optical unit having a plurality of light emission elements in a related art. In the figure, three light emission elements 41a, 41b, and 41c, a projection lens 42, and two dichroic mirrors 43a and 43b are disposed in a holder 40. The light emission element 41c is placed so that an optical axis LC of the light emission element 41c matches an optical axis LX of the projection lens 42. The light emission element 41b is placed so that an optical axis LB of the light emission element 41b crosses the optical axis LX of the projection lens 42 at right angles. The light emission element 41a is placed so that an optical axis LA of the light emission element 41a crosses the optical axis LX of the projection lens 42 at right angles.
If the light emission element 41a is turned on, light emitted from the light emission element 41a is reflected on the dichroic mirror 43a and is transmitted through the projection lens 42. If the light emission element 41b is turned on, light emitted from the light emission element 41b is reflected on the dichroic mirror 43b and the reflected light penetrates the dichroic mirror 43a and is transmitted through the projection lens 42. If the light emission element 41c is turned on, light emitted from the light emission element 41c penetrates the dichroic mirrors 43b and 43a and is transmitted through the projection lens 42.
In the example in the related art, however, if the optical path lengths from the light emission elements 41a, 41b, and 41c to the projection lens 42 differ, different focal lengths of the projection lens 42 result. Thus, the total of the optical path length from the light emission element 41a to the dichroic mirror 43a and the optical path length from the dichroic mirror 43a to the projection lens 42, the total of the optical path length from the light emission element 41b to the dichroic mirror 43b and the optical path length from the dichroic mirror 43b to the projection lens 42, and the optical path length from the light emission element 41c to the projection lens 42 are set almost equal to each other considering the light wavelength.
Therefore, the distance from the light emission element 41a at a position near the projection lens 42 to the optical axis LX of the projection lens 42 becomes long as compared with the distance from the light emission element 41b at a position distant from the projection lens 42 to the optical axis LX of the projection lens 42. Resultantly, the width of the optical unit, L0, increases and it is difficult to miniaturize the unit.